Carbon nanoparticles have received a great deal of attention since the discovery of the C60 buckminster fullerene molecule (H. W. Kroto, J. R. Heath, S. C. O'Brien, R. F. Curl and R. E. Smally, Nature 318, 162 (1985)) and the carbon nanotube (S. Ijima, Nature 354, 56 (1991)). Carbon nanoparticles are typically 1 to 100 nm in at least one dimension, carbon nanotubes however being up to a few millimetres in length. The explosion in C60 research in the early 1990s was driven by the production of large quantities (few milligrams) of the material by Krastchmer et al. (W. Kratschmer, L. D. Lamb, K. Fostiropoulos and D. R. Huffman, Nature 347, 354 (1990)) using a high pressure arc discharge method.
The remarkable mechanical and electronic properties exhibited by carbon nanotubes have encouraged efforts to develop mass production techniques. As a result, carbon nanotubes are becoming increasingly available, and more attention from both academia and industry is focused on the application of carbon nanotubes in bulk quantities. These opportunities include the use of carbon nanotubes as a conductive filler in insulating polymer matrices, and as reinforcement in structural materials. Other potential applications exploit the size of carbon nanotubes as a template to grow nano-sized, and hence ultra-high surface-to-volume ratio, catalysts or aim to combine carbon nanotubes to form nano-electronic elements.
The high cost and low production volume of carbon nanotubes are at present prohibitive for their use as a filler material in most large-scale structural and electrical applications. Presently, several industrial and governmental projects are underway to mass-produce several kilograms of single and multi-walled carbon nanotubes in a cost-effective manner.
Carbon nanotubes have been produced previously using various approaches including the laser or arc-discharge ablation of a carbon/catalyst mixture target. For larger scale synthesis, the most promising methods have been based on chemical vapour deposition (CVD). CVD typically uses a cheap feedstock and has relatively low energy requirements, and has therefore attracted interest for the purposes of bulk synthesis. In CVD methods, a carbon-containing gas is decomposed at high temperatures in the reaction zone of a furnace under the influence of a finely divided catalyst (usually iron, nickel, cobalt or other transition metals or alloys).
Catalyst particles may be manufactured in situ by the decomposition of metalloorganic compounds or may be introduced into the CVD furnace on a fixed substrate (W. E. Alvarez et al., Carbon 39 (2001) 547-558; WO00/17102; WO00/73205). For the growth of small nanotubes and single-walled nanotubes in particular, very small metal clusters (around 1 nm) are required.
Current CVD processes have the disadvantage that growing fibres condense at the low temperature region downstream of the reaction zone to form highly cross-linked networks. These networks block the flow of the gaseous carbon source, typically within 1 to 2 minutes from the start of the process. The blocking of the gas flow leads to a significant change in pressure and chemical composition at the reaction zone, with the result that the structure of the products is changed and the overall yield is reduced. This means that to achieve good results the products must be removed regularly, so that a continuous process is not possible.
In addition, the quality of the carbon nanotube products produced in this way is not controlled. Large carbon particles, amorphous carbon and thick diameter fibres are typically produced.
It is desirable to produce carbon nanotubes in the form of fibres or other agglomerates for ease of handling, or making objects in desired shapes or coatings on components for direct applications.
Attempts have been made to process cross-linked carbon nanotube networks into carbon nanotube fibres by dispersing the networks in an organic solution and drying the solution. The fibrous product thus obtained is a composite of carbon nanotubes and polymer (Brigitte Vigolo, Alain Penicaud, Claude Coulon, Cedric Sauder, Rene Pailler, Catherine Journet, Patrick Bernier and Philippe Poulin, “Macroscopic Fibres and Ribbons of Oriented Carbon Nanofibres”, Science 290, 1331 (2000)). Coagulation spinning of fibres from carbon nanotubes is also reported in US 2002/0113335 A (Lobovsky et al.).
It has been shown that a 30 cm long fibre of carbon nanotubes could be drawn from a network on a silicon substrate (Kaili Jiang, Qunqing Li, Shoushan Fan, “Spinning continuous carbon nanotube yarns” Nature 419, 801 (2002)).
Recently a 20 cm long fibre of single-walled carbon nanotubes was observed in the products of a CVD process (H. W. Zhu, C. L. Xu, D. H. Wu, B. Q. Wei, R. Vajtai and P. M. Ayajan, “Direct Synthesis of Long Single-Walled Carbon Nanotube Strands”, Science, 296 (2002) 884-886). However, this document does not disclose how to control the process to produce such fibres in high yield. The present inventors have found that the products produced in this way contain a high proportion of soot, and the fibres were obtained only in a small fraction of the products with a much narrower synthesis conditions window.